1. Field of the Invention
The present invention relates to a readily fibrillatable fiber comprising a vinyl alcohol polymer (abbreviated as "PVA" hereinafter) and a vinyl polymer different from the PVA, preferably an acrylonitrile polymer (abbreviated as "PAN" hereinbelow), characterized in that the fiber is readily modified into a super-fine fibril through the single action of chemically expanding force or mechanical stress or the combination thereof and is therefore preferable for use in wet-type or dry-type nonwoven fabric, friction material and reinforcing fiber for cement and rubber.
2. Description of the Prior Art
As the reinforcing fiber of a variety of friction materials for use in automobile brakes and clutch plates, conventionally, use has been made of asbestos, frequently, in terms of the trapping performance of inorganic micro-particles, thermal resistance, heat fusion resistance, reinforcing properties and the like. However, the use of asbestos has been put under strict regulations because of concern that asbestos may be harmful for human health. In recent years, therefore, the fibril of costly aramide fiber has been selected as an alternative thereof. However, aramide fiber is so costly that it is also used in a limited fashion. Thus, materials with insufficient reinforcing performance, such as natural pulp, are used as a kind of filler in combination, but such filler materials deteriorate the performance of the resulting products. Hence, a readily fibrillatable fiber has been desired, which is less expensive than aramide fiber and which has fibrillating properties so as to procure particle trapping performance, heat fusion resistance and reinforcing properties in combination.
Because asbestos is now under very strict regulations from the reason described above also in the field of slate plate and the like where asbestos has been used for reinforcing cement, general-purpose fibers such as vinylon are therefore currently used as an alternative of asbestos. Because general-purpose fibers are of a larger size than the size of asbestos, the resulting reinforced slate has such a lower green strength that the slate should be mixed with the fibril of natural pulp. These thick fibers and natural pulp deteriorate the product performance. Thus, the development of a fibrillatable fiber of a greater strength has been demanded. For reinforcing rubber and the like, metha- or para-aramide fiber or vinylon has been used, but such fiber is also problematic in that the fiber should be treated with resorcin-formaldehyde-latex process so as to enhance the adhesivity. Hence, a readily fibrillatable fiber of a higher adhesivity, a larger specific surface area and a greater strength has been required.
For satisfying these demands, a great number of attempts have been made to apply the phase separation phenomenon of blend polymers to a method for producing a super-fine synthetic fiber. For example, Japanese Patent Publication 10617/1974, Japanese Patent Publication 17609/1976 and Japanese Patent Application Kokai (Laid-open) 56925/1973 describe individually that a fibril can be generated by beating a sea-islands fiber comprising PAN as the sea component and PVA graft-copolymerized with acrylonitrile or a poly (methyl methacrylate) polymer as the islands component.
According to the techniques described in these publications, however, a mixture of water and the solvent of a stock solution or a single organic solvent having the solidifying potency is used in a solidifying bath because PAN is the sea component. However, not any uniform gel yarn can be recovered from such techniques because of the strong solidifying action, involving difficulty in higher drawing. Hence, it is difficult to produce a fiber with excellent reinforcing performance and a higher strength, in a stable and inexpensive manner industrially.
Because the compatibility of the polymers is so enhanced with the addition of a graft polymer that the spinning solution turns transparent to hardly form a phase separation structure, not only fibrillation then gets hard but also the resulting fibril is too thin and so readily tangible, involving a problem in that a fiber ball may readily be formed.
Furthermore, Japanese Patent Publication 31376/1972 discloses a readily fibrillatable PVA fiber comprising a completely saponified PVA as the sea component and a partially saponified PVA as the islands component, but the fiber has drawbacks such that the partially saponified PVA is solubilized during the beating process in water so that a specific system or a specific chemical reagent will be needed so as to treat the water used for the beating; that an increased volume of foam may be generated during the beating, which disturbs the beating process; and that the beatability of the fiber is not essentially satisfactory because the sea component and the islands component both comprise PVA polymers.